This invention relates to a process of continuously measuring and controlling the pH of an acid liquid, wherein the pH is continuously measured with a glass electrode.
Processes of continuously measuring and controlling the pH with use of glass electrodes are known. In those cases a dual two-phase system comprising glass as a common phase is described as a glass electrode. The voltages which depend on the ion activities are generated on the interfaces of the glass phase, which may also be described as a glass membrane, differently from the usage in electrochemistry. The action of the glass electrode is due to the fact that certain glasses assume with respect to aqueous solutions a potential which depends on the activity of the hydrogen ions in the solution. It is presently believed that that interfacial potential is due to an ion exchange process between the alkali ions of the glass and the protons of the solution in the gel layer developing on the glass membrane. The glass electrode can be used to determine the pH of solutions having a pH between 2 and 12. Outside that range the indicated potential is no longer directly proportional to the hydrogen ion concentration of the solution because errors of the readings in the acid or alkaline regions may occur. Such errors are caused by the chemical composition of the electrode glass. An error of the reading in the acid region will simulate a higher pH and an error of the reading in the alkali region will simulate a lower pH. For this reason the measurement and control of the pH of an acid liquid having a fluctuating pH will not be without problems, particularly if the fluctuating pH of an acid liquid may drop below 2, because this may result not only in an error in the acid region but also in a destruction of the glass electrode.
Published German Application 41 17 382 discloses a process of controlling the pH of an acid scrubbing liquid, which is used at a pH&lt;3 in a scrubbing zone. Acid constituents are removed from flue gas in the scrubbing zone and the scrubbing liquid is circulated at least in part. A partial stream of the scrubbing liquid is continuously mixed with a stream of an alkaline solution having a constant concentration and a constant volume rate and the pH of the mixture is continuously measured with a glass electrode. When the pH of the scrubbing liquid drops below 5, a neutral or alkaline correcting liquid is added to the scrubbing liquid until the pH exceeds 5. But that process has the disadvantage that even relatively small fluctuations of the relatively low pH of the scrubbing liquid will cause the pH at the electrode to change to a value above or below the range from 2 to 12 so that the pH cannot accurately be measured with a glass electrode.